This invention concerns an optical fiber that is used as a light source, for example, for wavelength division multiplexed optical transmission and with which a pumping optical pulse of a high peak output is made incident into the optical fiber to generate white light (super continuum light), which has a wide wavelength band at both sides of the pumping optical pulse.
With the development of optical communication technologies in recent years, wavelength division multiplexed optical transmission (WDM), with which a plurality of signal light, which mutually differ in wavelength, are multiplexed and transmitted through a single optical fiber, has come to be put to practical use. Wavelength division multiplexed optical transmission enables light of a plurality of wavelengths to be transmitted through a single optical fiber and is thus an optical transmission method that is suited for high-capacity, high-speed communication. Wavelength division multiplexed transmission is presently carried out by the application of an erbium-doped optical fiber type optical amplifier. Also, wavelength division multiplexed optical transmission is carried out in a wavelength band of 1.5 xcexcm, which is the gain band of the abovementioned optical amplifier.
In recent years, higher transmission speeds are being desired in optical communication that apply the above-described wavelength division multiplexed optical transmission. Light sources, using super continuum (SC) light, which is pulsed light of a wide wavelength width and substantially rectangular shape, are being noted as means for satisfying this demand. For example, Japanese Unexamined Patent Publication No. 90737 of 1998 proposes an optical fiber that generates the abovementioned SC light and a light source that uses this optical fiber.
SC light is generated when a pumping optical pulse of high peak power is made incident into a nonlinear medium having a dispersion decreasing zone, with which the wavelength dispersion changes, for example, from positive dispersion (anomalous dispersion) to negative dispersion (normal dispersion), along the length direction, from the side of incidence of light towards the side of exit of the light. The generation of SC light is a phenomenon in which an optical Kerr effect occurs as the abovementioned pumping optical pulse propagates through the abovementioned dispersion decreasing zone, causing wavelength broadening and pulse compression of the pumping optical pulse, and four-wave mixing and other nonlinear optical effects occur additionally to lead to the generation of a short pulse that is broadened to a wide wavelength band. When light of new wavelengths are generated by nonlinear phenomena at both sides of the wavelength of the input pumping light and these light of new wavelengths propagate through the nonlinear medium, new light are generated again by nonlinear phenomena at both sides of the wavelengths of the former new light. It is considered that the wavelength width of the spectrum is broadened by the repeated generation of such new light, leading to the generation of a substantially rectangular optical pulse with optical intensity over a wide bandwidth.
As has been mentioned above, SC light is white light that is generated in the wavelength bands at both sides of a pumping optical pulse of high peak power that is made to enter an optical fiber or other nonlinear medium, and this SC light may be obtained with a single pumping light source and a single SC light generating optical fiber. Thus by dividing the SC light by means of a wavelength division device, a plurality of light that mutually differ in wavelength may be obtained in a far more economical manner in comparison to methods that require the preparation of the same number of light sources as the number of signals to be transmitted.
Presently, a 1.5 xcexcm wavelength band erbium-doped optical fiber type optical amplifier is used for wavelength division multiplexed transmission. In the case where this optical fiber type optical amplifier is used to input pumping optical pulses into an optical fiber for SC light generation, SC light will be generated at the wavelength bands at both sides of the 1.5 xcexcm wavelength band. However, with prior-art optical fibers for wavelength division multiplexed transmission, absorption and an accompanying trailing edge occur near a wavelength of 1.4 xcexcm at the short wavelength side of the 1.5 xcexcm wavelength band due to OH groups that are incorporated in the process of manufacture of the optical fiber. When such a portion in which the optical fiber transmission loss is large exists in the wavelength range in which SC light is generated, the intensity of the generated light becomes attenuated during propagation, thereby obstructing the broadening of the wavelength width of the spectrum and the making of the spectrum rectangular in shape. Thus the wavelength width of the spectrum of SC light was limited at the short wavelength side of the 1.5 xcexcm wavelength band due to the abovementioned absorption, etc. by the OH groups.
Also, in generating SC light using an optical fiber, it is preferable that the polarized condition within the optical fiber not vary so that effective use can be made of the nonlinear phenomena within the optical fiber. The SC light generating optical fiber may thus be formed for example from a PANDA type polarization-maintaining optical fiber and thereby made to maintain the polarization. A PANDA type polarization-maintaining optical fiber is formed by providing a pair of stress-applying parts, which apply stress to the core, so as to sandwich the core from both sides. However, due to the effects of forming the stress-applying parts, SC light of adequate wavelength width could not be obtained from this type of PANDA type polarization-maintaining optical fiber even in the proposed example described above.
This invention has been made to solve the above problems. An object of this invention is to provide a white light generating optical fiber for wavelength division multiplexed transmission that has the following first to third characteristics. That is, the first characteristic of the optical fiber of this invention is that it can generate white light (SC light) that is adequately broadened in wavelength, the second characteristic is that the polarization is maintained so that nonlinear phenomena will occur efficiently, and the third characteristic is that the ill effects that accompany the maintaining of polarization can be avoided or lessened.
In order to achieve the above object, this invention provides the following arrangements as means for solving the problems. That is, the first arrangement of this invention is an optical fiber, which generates white light by means of nonlinear phenomenon in the wavelength bands at both sides of a pumping optical pulse that is input, and this optical fiber is characterized in being formed by covering the surroundings of the core by a first cladding, which is lower in refractive index than the core and covering the surroundings of the first cladding by a second cladding, which is higher in refractive index than the first cladding but lower in refractive index than the abovementioned core, by having, in at least part of the length direction of the optical fiber, a portion in which the wavelength dispersion gradient for the wavelength band at the wavelength side shorter than the wavelength of the pumping optical path is positive and the wavelength dispersion gradient for the wavelength band at the wavelength side longer than the wavelength of the pumping optical path is negative, by having, in at least part of the length direction of the optical fiber, a zone in which the wavelength dispersion for the wavelength of the pumping optical pulse varies from anomalous dispersion to normal dispersion from the side at which the pumping optical pulse is made incident towards the side at which light exits, and by having a transmission loss of the optical fiber that is 10 dB/km or less for light at a wavelength in the vicinity of 1.4 xcexcm.
The second arrangement of this invention is characterized in that, in addition to having the characteristics of the above-described first arrangement, the ratio (xcex942/xcex941) of the specific refractive index difference xcex941 of the abovementioned core with respect to the abovementioned second cladding and the specific refractive index difference xcex942 of the abovementioned first cladding with respect to the abovementioned second cladding is set in the range, xe2x88x920.4 to xe2x88x920.85, the ratio of the outer diameter of the abovementioned core to the outer diameter of the abovementioned first cladding is set in the range, 0.4 to 0.7, and the abovementioned specific refractive index difference xcex941 is set in the range, 0.6 to 1.2%.
The third arrangement of this invention is characterized in that, in addition to having the characteristics of the above-described first or second arrangement, the optical fiber has a polarization-maintaining mechanism that maintains the polarized condition of the light that propagates through the fiber.
The fourth arrangement of this invention is characterized in that, in addition to having the characteristics of the above-described third arrangement, the abovementioned polarization-maintaining mechanism is formed by making the cross-sectional shape of at least one of either the core or the first cladding an elliptical shape.
The fifth arrangement of this invention is characterized in that, in addition to having the characteristics of the above-described fourth arrangement, the: ratio of the major diameter to the minor diameter of the abovementioned elliptical shape is set in the range, 1.3 to 2.0.
The sixth arrangement of this invention is characterized in that, in addition to having the characteristics of the above-described third arrangement, the abovementioned polarization maintaining mechanism is formed by providing a pair of stress-applying parts inside the second cladding, which apply stress to the core, so as to sandwich the core from both sides and this pair of stress-applying parts is made axially symmetrical with respect to the abovementioned core.
The seventh arrangement of this invention is characterized in that, in addition to having the characteristics of the above-described sixth arrangement, the shortest distance between the abovementioned pair of stress-applying parts is 2.3 times or more the mode field diameter for the wavelength of the pumping optical pulse.
The eighth arrangement of this invention is characterized in that, in addition to having the characteristics of the above-described sixth arrangement, the diameter of the abovementioned stress-applying parts is 25 to 37 xcexcm.
The ninth arrangement of this invention is characterized in that, in addition to having the characteristics of the above-described first arrangement, the outer diameter of at least one of either the abovementioned core or the first cladding varies along the length direction of the optical fiber.
The tenth arrangement of this invention is characterized in that, in addition to having the characteristics of the above-described first arrangement, the incidence efficiency of the light that is made incident into the abovementioned optical fiber is greater than the exit efficiency of light that exits the optical fiber.
The eleventh arrangement of this invention is characterized in that, in addition to having the characteristics of the above-described first arrangement, the specific refractive index of at least one of either the abovementioned core or the abovementioned first cladding varies along the length direction of the optical fiber.
The twelfth arrangement of this invention is characterized in that, in addition to having the characteristics of the above-described first arrangement, at least the abovementioned core is formed by the VAD method.
This invention provides an optical fiber with which the wavelength dispersion characteristics and dispersion gradient characteristics are determined so as to enable white light to be readily obtained and with which the transmission loss characteristics at a wavelength of 1.4 xcexcm, which prevent wavelength broadening at the short wavelength side of white light that is generated at the short wavelength side of the 1.55 xcexcm wavelength band, are made small. This invention therefore provides an optical fiber that can generate white light (SC light) of an adequately broadened wavelength.
Also, with the second arrangement of this invention, the refractive index profile of the optical fiber and the ratio of the core diameter to the first cladding diameter can be optimized to provide an optical fiber that exhibits the abovementioned excellent effects.
When an optical fiber is used, having a portion in which the wavelength dispersion characteristic is substantially flat, and a pumping optical pulse, having a wavelength in the range of flat wavelength dispersion, is made incident into the optical fiber, SC light will be generated at both sides of the incident light. In order to generate SC light in a desired wavelength range, it is important to obtain an optical fiber with a portion which has zero wavelength dispersion and is flat in wavelength dispersion in the desired wavelength range.
With the second arrangement of this invention, the cladding has a two-layer structure and the refractive index profile thereof is a so-called W type profile. With this W-type profile, a portion, in which the dispersion is zero and the wavelength dispersion is flat (portion in which the wavelength dispersion is zero and flat) can be set in the desired wavelength range that includes the 1.5 xcexcm wavelength band. Since a quartz optical fiber has the lowest transmission loss at a wavelength of approximately 1.5 to 1.6 xcexcm, SC light can be generated in a wavelength range of low transmission loss by setting the abovementioned flat portion close to 1.5 xcexcm.
Though an optical fiber having a portion, in which the wavelength dispersion is zero and flat in the 1.5 xcexcm wavelength band, may be realized for example with an optical fiber with a four-layer cladding structure, such an optical fiber will be complex in structure. Thus when such a type of optical fiber is used, it will be difficult to accurately control the portion that is flat in wavelength dispersion at the zero dispersion wavelength. In contrast, the second arrangement of this invention enables a portion, which has zero dispersion and is flat in wavelength dispersion in a desired wavelength range that includes the 1.5 xcexcm wavelength band, to be set with a relatively simple arrangement, and it thus provides a structure that is best in terms of productivity.
Furthermore, with the third arrangement of this invention, since it becomes possible to stabilize the polarized condition of the light that propagates through the optical fiber, white light of good characteristics can be obtained.
Furthermore, with the fourth to sixth arrangements of this invention, a polarization-maintaining fiber for arranging an optical fiber that exhibits the abovementioned excellent effects can be formed readily by making the cross-sectional shape of at least one of either the core or the first cladding an elliptical shape or by providing stress-applying parts in an axially symmetrical manner so as to sandwich the core.
Furthermore, with the seventh and eighth arrangements of this invention, the interval and diameter of the stress-applying parts provided in the optical fiber are optimized. Thus with an optical fiber having these arrangements, the prevention of the broadening of white light (SC light) is restricted without fail by the provision of stress-applying parts and the polarized conditions in the respective wavelengths can be maintained along the length direction of the optical fiber. The seventh and eighth arrangements of this invention can thus provide an optical fiber with which white light of an adequately broadened wavelength can be generated even more definitely.
Furthermore, with the ninth through eleventh arrangements of this invention, since the dispersion characteristics can be varied along the length direction of the optical fiber by varying the outer diameter or the specific refractive index of at least one of either the core or the first cladding, an optical fiber for generating white light can be readily obtained.
Since the optical fiber of this invention has wavelength decreasing characteristics, the coupling efficiency to the optical fiber may differ at the incident and exit ends. However, with the tenth arrangement of this invention, since optimization is carried out so that the efficiency of incidence into the optical fiber is made greater than the efficiency of exit from the optical fiber, the pumping light from a pumping optical pulse source can be coupled to the SC light efficiently and SC light can thus be generated efficiently.
Furthermore, with the twelfth arrangement of this invention, since at least the core is formed by the VAD method, an optical fiber that exhibits the abovementioned excellent effects can be readily formed.